1. Field of the Invention
The invention relates to a fuel injector with direct needle control for an internal combustion engine.
2. Description of the Prior Art
Fuel injectors with a so-called direct needle control are known. Fuel injectors of this kind function properly without a control valve interposed between an electrically triggered actuator and a nozzle needle. The transmission of force between the actuator and the nozzle needle is implemented by means of a hydraulic coupler or hydraulic booster. For these actuators, it is particularly useful to use piezoelectric actuators, which have a direct or inverse triggering, depending on whether or not they are supplied with current in the closed state. With a direct triggering, the piezoelectric actuator is supplied with current in order to open the nozzle needle so that a longitudinal expansion of the piezoelectric actuator, through a pushing motion that is amplified by the booster, triggers an opening of the injection nozzles. In the closed state, the piezoelectric actuator has a shorter longitudinal span. With an inverse triggering, the piezoelectric actuator is supplied with current in the closed state of the nozzle needle so that when the piezoelectric actuator is in its longitudinally expanded state, it holds the nozzle needle closed. When the piezoelectric actuator is triggered to initiate the injection, the power to the piezoelectric actuator is switched off so that a pulling movement of the piezoelectric actuator causes a pressure drop in a control chamber of the hydraulic booster. This hydraulically boosts the stroke motion of the piezoelectric actuator in order to open the nozzle needle.
A fuel injector with direct needle control has already been proposed by patent application DE 10 2004 037 125.3. The fuel injector therein has an actuator booster piston and a nozzle needle booster piston; the actuator booster piston is associated with an actuator coupler chamber and the nozzle needle booster piston is associated with a nozzle needle coupler chamber. Between the actuator coupler chamber and the nozzle needle coupler chamber, a hydraulic throttle restriction is provided that has different flow cross sections for the flow of fuel into and out of the nozzle needle control chamber. A first sliding sleeve for delimiting the actuator coupler chamber is guided axially on the actuator booster piston and another sliding sleeve for delimiting the nozzle needle coupler chamber is guided axially on the nozzle needle booster piston. A compression spring prestresses the sliding sleeves so that they each press with an end surface against a respective sealing surface. The use of sliding sleeves makes it possible to axially decouple the actuator booster piston from the nozzle needle booster piston, permitting the booster pistons to be installed in axially offset positions.